1. Field of the Invention
This invention relates to the vapor-permeation curing of a resinous coating composition.
The invention is particularly applicable to curing coatings of special urethane compositions whose chemistry is described in U.S. Pat. Nos. 3,789,044, 3,833,226, 3,836,491, and Canadian Patent No. 1,005,943.
2. Description of the Prior Art
U.S. Pat. No. 4,105,725 discloses a method and apparatus for generating a steady supply of catalyst vapor, typically triethylamine (TEA) in an inert carrier gas such as nitrogen or carbon dioxide. The invention provides an improved process, operable at ambient temperature, under which catalyst vapor containing typically 4 to 7% TEA, from such a generator may be used to contact a wet vapor-curable film or coating on a substrate to bring about a rapid cure to a dry coating adhering to the substrate. Typically ambient temperatures by the use of the invention, are 70.degree. to 85.degree. F. and curing time 10 to 90 seconds.
A preferred use of the process is in treating a web, having on its surface a coating of catalyst-curable synthetic resin, through a vapor curing chamber. Catalyst vapor is injected into the chamber to create an atmosphere of the vapor in contact with the coating on the web passing through it and means is provided at the entry and the exit of the chamber to minimize the ingress or egress of gases.
It is well known, by analogy with momentum and heat transfer, that mass transfer will proceed at the most rapid rate when the flow is normal to the substrate as opposed to tangential. Hence, this design calls for impingement of catalyst vapor normal to the substrate. If this cannot be achieved, for some reason, then the design must provide for turbulent flow (high Reynolds number) of the catalyst vapor at the coating/catalyst interface.
One of the problems is that a web, on its passage into a treating chamber from the atmosphere, entrains with it a boundary layer of air which interferes with the access of the catalyst vapor to the coating. Furthermore, air carried into the chamber, by the web, tends to create an explosive atmosphere. Further the catalyst laden atmosphere tends to escape from the chamber which represents the principal cost of the process.
The present invention is aimed at overcoming these disadvantages and at providing certain positive advantages as will be evident from the following description.